Communication system



March's, 1934. E S 'REBY 1,950,127

COMMUNICATION SYSTEM Filed Feb. 25, 1932 2 Sheet-Sheet 1 FIG! A, l l l r l i 1l R @(Q E mm 5 7 e I 555% IN VE N TOR M. E. STR/EB) A TTORNEV PatentediMar. 6, 1934 COMLIUNICATION SYSTEM Maurice E. Strieby, Maplewood, N. l., assignor to Bell Telephone Laboratories, Incorporated, New York,'N. Y., a corporation of New York Application February 25, 1932, Serial No. 595,051

3 Claims.

This invention relates to conducting systems for the transmission of intelligence and more particularly to such systems wherein one conductor serves as a coaxial return for another.

8 An object of the invention is to reduce the disturbing effect of extraneous electric and magnetic fields on systems of the type to which the in vention relates. Another and more particular object is to reduce the effect of signal induction 10 between parallel conducting systems.

From another aspect, the object of the present invention is to make possible a reduction in the thickness of the return conductor of a coaxial system. Withoutpermitting interference to become second has been found to be one comprising a central conductor and a tubular or shell-like re- 2j(l turn conductor maintained in coaxial relation withthe first conductor and separated from it by a dielectric that I is substantially gaseous. The outer conductor, since it completely surrounds the central conductor and is normally grounded,

affords a high degree of shielding against external electric disturbances. External magnetic fields, because of the coaxial arrangement of the conductors, also have little effect on the system, at least at high frequency. Where the conductors are thick enough to be mechanically self-supporting the shielding may be so eiilcient in fact, that the extent to which signals may be attenuated is determined not by the. level of static and crosstalk as in other systems but only by the resistance noise of the conducting material and interference from other sources inherent in the system.

In some situations external sources of interference may be so powerful or so near the signaling line that the level of induced disturbance be comes objectionably high. Such a condition might be found, for example, where the trans mission line passes in. the immediate vicinity of a powerful radio broadcasting station. Where also a multiplicity of coaxial conductor pmrs are brought together to form a cable, as disclosed in an application for patent bearing Serial No. 487,153 filed by H. W. Dudley on October 8, 193b, induction or crosstalk from one circuit to another becomes a serious consideration, and in some 0 cases it determines the frequency range may be utilized. Increasing the thickness of the outer conductor alleviates the diiilculty, but as a practical matter it is not very feasible, since the flexi-= bility of the cable is thereby reduced and the costbi. the conducting material, already the major item of expense in a system oi this type, is in creased.

In accordance with the present invention it is proposed to reduce the efiect of external fiields on a coaxial conductor transmission line by electrically transposing the conductors of the pair at intervals along the line. Where a number of coaxial pairs are assembled as a cable, such electrical transposition is applied to the several circuits in accordance with a regular system, analoas gene to the system used for the physical transposition oi open wire lines. Since the outer condoctor of a coaxial pair is normally grounded, an

ordinary transposition of a signaling circuit would result in a short circuit. It is sufficient to achieve ii-2) the effect of a transporition, however, that the phase of the signals in the central conductor be reversed at intervals, and for this purpose it is proposed to insert phase reversing devices, such as transformers, at the points where transpositions it; are desired.

A subsidiary object of the present invention is to provide a system wherein power for energizing the elements of repeating apparatus may be supplied over pair of coaxial conductors in which so electrical transposition is utilized.

The nature of the present invention will appear more fully in the following detailed description of several preferred embodiments. Reference will be made to the accompanying drawings, in 35 which:

i. represents a typical coaxial conductor system with transpositions in accordance with the present invention; Fig. 2 shows a modification or a detail thereof;

.Fig. 3 represents a multi-circuit coaxial conductor cable; Fig. shows a suitable scheme of transpositionstherefor;

Figs. 5 and 6 show schematically a system for supplying alternating current to repeaters over a transmissionline in which electrical transpositions are present; and Figs. 7 and 8 show similar direct-current systems.

Referring now to Fig. 1, there is shown a portion of a communication system embodying the 1 present invention. At A is represented a terminal station which may be adapted to generate and receive carrier wave telephone or other signals aging in frequency from perhaps five hundred ...J cycles per second to five megacycles per secend. Signals to and from the terminal station transmitted over a line comprising a central conductor 1 and a hollow outer conductor 2, which are maintained in coaxial relation by means of a suitable separating structure. At in- 119 tervals of several miles repeater stations -R are inserted in the line. The particular nature of the terminal and repeater circuits and the specific type of coaxial conductor line employed is of no consequence as regards the present invention. Suitable embodiments are disclosed, for example in H. A. Affel, Patent 1,835,031, Decemher 8, 1931.

The transmission line is divided into a plurality of sections L1, L2, Ln, etc., by means of transformers T1, T2, Tn, etc., which convey signals from one section to the next. Thus, the central conductor 1 of the first section L1 is connected to one terminal of winding 5 of transformer T1. The other terminal of winding 5 is connected to the outer conductor 2, which is at ground potential. Winding 6 of the same transformer is similarly connected to section-Lz of the line, one terminal being connected to the central conductor of that section and the other to the grounded outer conductor. The polarity of the windings of the several transformers, as indicated by the solid arrows, is such as to produce a reversal in phase of signal current in successive sections of line. That is, if current in the central conductor of section L1, for example, were flowing in'the direction of repeater R, as indicated by the solid arrows, the current in the central conductor of the next section L2, would be caused to flow away from the repeater.

The neutralizing effect of the transposition on waves induced from external sources can readily be seen. If the several sections of line are affected by the same external source of disturbance, the waves induced in the central conductor will be in the same direction or sense in all ofthem, as represented by the dotted arrows. The disturbing waves induced in one section, as Ll, for example, are then transmitted to the adjacent section L2, but reversed in phase because of the polarity of the transformer windings. As the waves induced in the latter section from the external source and those conveyed to it by the transformer are opposite in phase they tend to neutralize each other and their resultant effect in the circuit is diminished.

The number of transpositions required in each repeater section and their location will depend on several factors, including the nature, proximity, strength and position along the line of the disturbing sources. Generally, a given source of interference will affect only a relatively small portion of the line and hence only that portion may require transposition.

In Fig. 2 is shown one method for shielding the transformers against external disturbances. The transformer 'I is shown enclosed in a shielding container S which in effect is a mere enlargement of the outer conductor. Where it is feasible, the container may be dispensed with and the transformer located within the outer conductor itself. Preferably, a grounded shield is placed between the windings of the transformer.

In Fig. 3 is represented a cable 12 comprising a a multiplicity of coaxial conductor pairs 15. Each pair may be associated with suitable terminal and repeating apparatus for transmitting carrier signals in a frequency band of, for example, fifty to five hundred kllocycles per second. A more detailed disclosure of cable and circuits is unnec-.

essary here but reference is made to the patent application of H. W. Dudley, supra. At intervals along the cable, conductor pairs are interrupted and transformers are inserted to effect transpositions. Coaxial conductors are to be preferred for making the necessary connections. The transformers should give a unity impedance ratio and introduce as little loss as possible. As a specific example of a suitable type, the transformer may have a toroidal core built up of permalloy ribbon two mils thick and primary and secondary windings thereon of approximately one hundred turns each. An electrostatic shield may separate the windings.

In amulti-pair cable all of the pairs can not be transposed at the same point, since, while this would reduce interference from sources external to the cable, it would have no effect on inter-circuit induction or crosstalk. Instead, a regular system of transpositions is employed, wherein at any one point only two or three of the circuits, as 13 and 14, may be provided with transformers S1, S2, respectively. At each transposition point a suitable container or pot may be used to house the several transformers, similar, for example, to the structures used for loading coils.

A typical transposition system for a six-pair cable is shown in Fig. 4. Between the terminals A and G of a repeater section, one line, as Z1, may be transposed at only two points, while another, as lg, may be transposed as many as five times. In general, the scheme provides that the number of transpositions in any one pair is half or twice that of adjacent pairs. The scheme can be extended, of course, as the number of .pairs in a cable requires.

Referring now to Fig. 5, there is shown a transmission system including a terminal station A, a repeater station R, and a transmission line L connecting them, transformers T1 Tn, etc., being provided at intervals along the line to effect transpositions. An alternating current generator G, which is connected to the line at station A, supplies current over the line to repeater R and to subsequent repeaters. A power supply unit 29 associated with each repeater is adapted to convert the alternating current received to a form suitable for energizing the vacuum tubes or other elements of the repeater.

At transposition points means are provided for shunting the power current around the signal transformers Tl, Tn, etc., since the latter are not adapted to transmit either large amounts of energy or power frequencies. This means may take the form of power transformers PTi. PTn, etc. connected effectively in parallel with the signal transformers. High-pass filters 22 and low-pass filters 23 of simple design may be connected in the leads of the two transformers to prevent detrimental inter-action. A similar filtering arrangement may be used at terminal and repeater stations. Thus, the signal input circuits at repeater R are connected to the preceding section of line through high-frequency filters 25 and 26, While the power supply unit 29 is connected to the line through low-pass filter 35. Power is supplied to the succeeding line section through winding 33 of power transformer 31 and low-pass filter 34.

The range of frequencies that a coaxial conductor line is capable of transmitting efficiently may be, as hereinbefore noted, from zero to five megacycles per second. Repeaters, as ordinarily designed, are not capable of amplifying effectively all frequencies of such a hand. For car rier telephonesystems, the lowest frequency of the band to be amplified by one amplifier. can

not practically be made much less than one-tenth fled, then, the lowest frequency for the same amplifier should not be less than approximately five hundred kilocycles per second. Repeater 28 of Fig. 5 and the associated band-pass filters 26 may accordingly be designed to amplify and pass this range of frequencies. The frequency range below this five hundred kilocycle lower limit may also be utilized if a suitable repeater is provided. Repeater 27 and its associated band-pass filters are therefore designed to amplify and pass a band of from approximately fiIty kilocycles per second to five hundred kilocycles per second. The line attenuation in this low-frequency range is much less than in the high-frequency range. The amplification required of repeater 27 is therefore less than that of repeater 28. Alternatively, if high gain is used, a 50-500 kilocycle repeater need be used at perhaps only alternate repeater stations. Where alternating current is not transmitted over the line to energize the repeaters, filters 25 and 26 may be of low-pass and highpass types, respectively.

A typical repeater adapted to be supplied with alternating current power over the signaling line is represented schematically in Fig. 6. Power currents arriving over the line are separated by low-pass filter and applied to the primary winding of power transformer 37. The terminals of the secondary winding 38 of transformer 37 are connected through a low-pass filter 34 to the next section of the signaling line. The secondary winding 38 is tapped at an intermediate point to supply a suitable voltage to rectifier-filter 39, which supplies current to the space paths of discharge devices V1 and Va. A low voltage tap is provided for the heater elements of the discharge devices. Incoming signal currents are selected by filter 32, which may be of the high-pass type, and applied to input transformer Tl, whence they pass through the amplifier, output transformer To and filter 36 to the outgoing line.

Where direct current is to'be supplied over the signaling line to the repeaters the system shown in- Fig. 7 may be used, or where two or more pairs .1 of lines are present, that represented in Fig. 8.

In the system shown in Fig. 7', a battery orother source of direct current '42 is connected across the line at the terminal station A. A blocking condenser 41 prevents short circuit of the battery by the terminal circuits. At transposition points, the transformers T1, Tn, etc. used for transposition purposes are bridged by a choke coil 44 orv other suitable device which will pass direct current to the exclusion of signaling fre quencies. The return path for-direct current is provided by one side of the signaling line, which may be continuous and grounded as in the system shown in Fig. 1. A choke coil 48 is similarly used at the repeater station R to convey direct current from one, repeater section to another. Any suitable unit may be used at the repeater station for applying the direct current diverted to the elements of the repeater.

If two or more signaling circuits are available, as in the cable system of Fig. 3, separate filament and anode supply circuits may be used. Thus, in Fig. 8 an A or filament supply battery is connected across line L1 at the terminal station and a "3" battery across line L2. At the common repeater station R, each line is provided with an in- 1 dividual amplifier, vacuum tubes V1, V2, and Va being associated with line L1, and V4, V5 and V0 with line In. Filament heating current for all tubes is supplied from line L1. In the specific circuit shown, the central conductor 1 of the coaxial conductor line L1 is connected through a choke coil 51 and the filaments of V1 to V; connected in series, to the central conductor of the outgoing section of line. The filament circuit may include similar elements at succeeding repeater stations and it is eventually completed by connection to ground. Space current for all tubes is supplied over line L2. A continuous diroot-current path through the repeater is provided'by choke coil 52 which connects the central conductor of one section to the central conductor of the next section. The grounded outer conductor provides the return path of this directcurrent circuit. Space current for the vacuum tubes is obtained by connecting the anodes of each amplifier together and through a choke coil .55, 56, individual to the amplifier to the central conductor of line L2. The signaling circuits of the amplifiers, which are shown incompletely, may be of suitable conventional design.

The present invention may of course be embodied in other forms than the ones described herein for purposes of illustration. It is therefore to be limited only by the scope of the appended claims.

What is claimed is:

1. A transmission system comprising a pair of coaxial conductors connected one as the return for the other, means for applying to, said conductors bands of signaling waves occupying a wide range of frequencies, means for successively reversing the phase of said signaling waves throughout a region where said conductors are exposed to disturbing fields the outer of said conductors being so thin and the minimum signaling frequency being so low'that excessive interference would be created in the lowermost of said signaling bands except for said phase reversing means, and a shielding structure electrically continuous with said outer conductor for enclosing each of said phase reversing means.

2. A combination in accordance with claim 1 in which said phase reversing means comprise 1 transformers, the primary and secondary windings of said respective transformers being electrostatically shielded from each other.

3. In a multiplex carrier telephone transmission system, a cable. comprising a multiplicity of pairs of coaxial conductors, the outer conductors of -said pairs being insufliciently thick to prevent excessive mutual inductive eifects between said pairs, signal repeaters at intervals along said cable, means for electrically transposing the conductors of each of said pairs at a plurality of points between said repeaters, the transposition interval for one pair of conductors being diiierent from that of pairs adjacent to it in said cable, and respective shields electrically continuous with the outer conductor of each of said pairs for enclosing said electrical transposing means.

MAURICE E. S'I'RIEBY. 

